Hi John
Noise in a pwm driven motor is due to movement of metal parts in the motors magnetic field. The emitted level depends on the quality of construction better varnish fill, tightnes of the bearings and finaly the natural resonance characteristics of the motor itself.
The useful pwm frequency upper limit while to some extent aplication driven is for practical purposes controlled by the loss of torque from the motor. As you raise the pwm frequency above the limit the torque drops uniformly across the entire operating range.

If as you change the pwm frequency the emited noise level suddenly rises you are seeing resonance and do not want to operate at that frequency as it will shorten the motors life.
Also higher pwm frequencys within the practical range increase the heating due to hysterisis losses in the iron parts. hth Terry